Thanks for all the interesting thoughts.
I thought I'd update this (the dinner is tomorrow night).
I thought of doing another cab based wine from the same year, either another Bordeaux or from somewhere else.
I thought of Dominus, or Sassicaia, but rejected them. I am really first and foremost a student of Bordeaux and have been for many years.
I settled on going for a vertical. I will be bringing a 1982 Leoville Poyferre to supplement the 1990 provided by the host. Two excellent vintages and a chance to review two different stages of evolution was too much to pass up.
Poyferre had more or less sat on the back benches for many years. They had even managed to tank excellent vintages like 1964 and 1970 (though they shared that, surprisingly, with Las Cases). Take a look at a set of RP reviews some time. An unremitting series of scores in the 70s and 80s made Poyferre not only an also-ran, but a never-will-run-again in the minds of most claret fans.
Then in 1982, mirabile dictu, a home run out of literally nowhere. Then a lesser but still good 1983, and after that the return of the Great Slump until - 1990, another spectacular wine. Since then, much more reliability, with good wines in every good vintage (not so much in off vintages) and more home runs in 2000, 2009 and 2010.
The temptation to taste the first two great wines they accomplished while still stumbling toward former glory was too tempting and so that's what I decided to offer.
I'll post notes and let you know how it turns out.